Spectral colorimetric apparatus and image forming apparatus using the same

ABSTRACT

A spectral colorimetric apparatus for detecting a color of an image of a subject, including: an illumination optical system illuminating the subject on a detection surface; a spectral optical system including a spectral element spectrally separating the beam diffused by the subject and a light receiving element array detecting a spectral intensity distribution; and a guiding optical system for guiding a beam diffused by the subject, wherein: the detection surface is parallel to a spectral plane including a principal ray of a beam entering the spectral optical system and a principal ray of a beam spectrally separated; the principal ray of the beam enters the spectral optical system within the spectral plane obliquely to a line joining a center of the light receiving element array with a surface vertex of the spectral element; and a light receiving surface of the light receiving element array is orthogonal to the spectral plane.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a color sensor using a diffractiongrating for color recognition or colorimetry on a subject and, moreparticularly, is suitable for a color sensor which is incorporated in acolor image forming apparatus to perform colorimetry on toner orprinting media.

2. Description of the Related Art

In an image forming apparatus for forming a color image through anelectrophotographic process, a deviation in color tone may occur becauseof color mixing of toner. The same problem occurs, not only in theelectrophotographic type image forming apparatus but also in generalimage forming apparatuses for forming a color image such as an ink-jettype image forming apparatus. In the electrophotographic type,particularly, drum sensitivity, the charge capacity of toner, andtransfer efficiency to a paper sheet are changed for each colordepending on the condition of environment, and hence a color mixingratio is deviated from a predetermined value and thus it is highlylikely to affect a color tone. Further, there is a need for therealization of higher-degree of color tone reproduction in the printindustry.

In order to solve the problems described above, Japanese PatentApplication Laid-Open No. H09-160343 proposes a method of measuring aspectral reflection light quantity of a toner image using two differentspectral filters and correcting an image signal based on the result ofthe measurement. Similarly, Japanese Patent Application Laid-Open No.2004-126278 proposes an image forming apparatus for performing colortint correction on a color image. In the electrophotographic type imageforming apparatus, according to Japanese Patent Application Laid-OpenNo. 2004-126278, a color tint detection sensor is provided in thedownstream of a fixing device located on a fixing and conveying path todetect RGB output values of colors of a mixed color patch image formedon a transferring material (printed image) moving along a conveyingpath.

In order to determine the color tone at higher precision, it isnecessary to increase the number of spectral wavelength bands to atleast three, which corresponds to the number of primary colors. When thenumber of wavelength bands may be further increased, the color tone maybe determined at higher precision. In order to increase the number ofwavelength bands, there have been proposed a large number of diffractionspectral devices for performing spectral measurement based on adiffraction phenomenon, for example, in Japanese Patent ApplicationLaid-Open No. H06-058812, Japanese Patent Application Laid-Open No.H06-050814 and Japanese Patent Application Laid-Open No. 2001-264173.

An example of a colorimetric device capable of measuring absolutechromaticity at high wavelength resolution is a spectral colorimetricapparatus for obtaining chromaticity from a spectral intensitydistribution using a diffraction spectroscope.

FIG. 9 illustrates a Rowland type diffraction spectral colorimetricapparatus generally used as a conventional diffraction spectral device.

A light beam to be detected is entered into a detection opticalapparatus from an incident window 108. Specifically, scattering lightfrom a subject illuminated by an illumination optical system (not shown)is entered into the detection optical apparatus from a stop. The lightis spectrally separated by a concave reflective diffraction element 104and then obtained as a spectral intensity distribution by aone-dimensional array detector 103.

In a color image forming apparatus for forming a color image asillustrated in FIG. 8, the spectral colorimetric apparatus is requiredto be incorporated for use into the main body of the color image formingapparatus. In practice, a reduction in size of the spectral colorimetricapparatus in a conveying direction of a printed paper sheet, that is, ina direction perpendicular to a detection surface is strongly demanded soas not to hinder the conveying of the printed paper sheet in the colorimage forming apparatus.

The image forming apparatuses disclosed in Japanese Patent ApplicationLaid-Open No. H09-160343 and Japanese Patent Application Laid-Open No.2004-126278 employ a colorimetric device using an RGB filter. Thecolorimetric device is small in size but the number of wavelength bandsis small, and thus is not suitable to measure accurate absolutechromaticity. Further, when the number of filters is to be increased,there is a problem that a significant increase in cost occurs.

In the case of a Rowland type spectral device, which is generally usedas the diffraction spectral device, imaging magnification effected bythe diffraction grating shows substantially qui-magnification, that is,the ratio of the size of the stop and the image thereof on the arraydetector are substantially 1. When a Rowland circle is made smaller, thespectral unit may be relatively easily reduced in size. However, whenthe entire structure including the illumination system is taken intoaccount, there is still room for improvement. In the device disclosed inJapanese Patent Application Laid-Open No. H06-058812, the detectionsurface and the spectral plane are perpendicular to each other, andhence the device is increased in size in a direction perpendicular tothe detection surface. In the device disclosed in Japanese PatentApplication Laid-Open No. H06-050814, a test subject and a stop are madeconjugate with each other so that a light beam to be detected which isemitted from the test subject forms an image on a stop through a mirrorand an imaging lens. The optical path of the light beam to be tested isfolded by the mirror to reduce the device in size. However, thedetection surface and the spectral plane are made perpendicular to eachother, and hence the device is increased in size in the directionperpendicular to the detection surface.

There is another problem that, when the entire apparatus including theillumination system is reduced in size, it is difficult to remove flarelight from the illumination system. When the flare light reaches asensor, a noise is superimposed on an output of the sensor, making itdifficult to perform accurate chromaticity measurement.

SUMMARY OF THE INVENTION

The present invention has been bade in view of the problems describeabove, and therefore, it is an object of the present invention toprovide a spectral colorimetric apparatus which may be significantlyreduced in size in a direction perpendicular to a detection surfacewhile effectively reducing a noise caused by flare light due to thereduction in size and is suitable for an image forming apparatus.

In order to attain the above-mentioned object, the present inventionprovides a spectral colorimetric apparatus for detecting a color of animage of a test subject illuminated, including; an illumination opticalsystem for illuminating the test subject located on a detection surface,a spectral optical system including a spectral element which spectrallyseparates the light beam diffused by the test subject and a lightreceiving element array which detects a spectral intensity distribution,and a guiding optical system for guiding a light beam diffused in thetest subject, in which; when a plane including a principal ray of alight beam incident on the spectral optical system and a principal rayof a light beam having been subjected to the spectral separation by thespectral element is defined as a spectral plane, the detection surfaceand the spectral plane are parallel to each other, the principal ray ofthe light beam enters the spectral optical system within the spectralplane enters obliquely with respect to a straight line joining a centerof the light receiving element array with a surface vertex of thespectral element, and the light receiving surface of the light receivingelement array and the spectral plane are orthogonal to each other.

According to the present invention, the thickness of the spectralcolorimetric apparatus in the direction perpendicular to the detectionsurface may be reduced to a value significantly smaller than that of aconventional spectral apparatus. Further, when the spectral colorimetricapparatus is combined with a Rowland type spectral system, a maximumreduction in size may be achieved by a minimum structure. Therefore, thespectral colorimetric apparatus is easily incorporated in an imageforming apparatus or the like, and hence a paper sheet conveying unitwhich is usually located after a fixing device may be made more compact.In this manner, the spectral colorimetric apparatus also contributes toreduce the size of the image forming apparatus. Still further, thespectral colorimetric apparatus is reduced in thickness, and hence theconveyance of the paper sheet is not hindered. Thus, high-precisionchromaticity measurement may be performed without changing a processspeed.

Further, the illumination light source and the light receiving elementarray are disposed with a reference axis of the spectral unit sandwichedtherebetween, and hence a noise caused by direct flare light from theillumination optical source may be significantly reduced, which allowsstable and high-precision chromaticity measurement to be performed.Therefore, more stable and high-precision color reproduction may beachieved by the image forming apparatus.

Further features of the present invention become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating a spectral colorimetricapparatus according to Embodiment 1 of the present invention.

FIG. 2 is an internal perspective view of the spectral colorimetricapparatus according to Embodiment 1 of the present invention.

FIG. 3A is a principal cross sectional view (taken along the line 3A-3Aof FIG. 2) illustrating an illumination optical system of the spectralcolorimetric apparatus according to Embodiment 1 of the presentinvention.

FIG. 3B is a principal view illustrating the illumination optical systemof the spectral colorimetric apparatus according to Embodiment 1 of thepresent invention illustrated in FIG. 3A viewed in Y-axis direction.

FIG. 4 is a graph illustrating a variation in light quantity of theillumination optical system of the spectral colorimetric apparatusaccording to Embodiment 1 of the present invention, depending on avariation in test subject position (height direction).

FIG. 5 is a principal cross sectional view (taken along the line 5-5 ofFIG. 2) illustrating a guiding optical system of the spectralcolorimetric apparatus according to Embodiment 1 of the presentinvention.

FIG. 6A is a principal cross sectional view illustrating a spectral unitof the spectral colorimetric apparatus according to Embodiment 1 of thepresent invention.

FIG. 6B is a schematic view illustrating a concave surface reflectiontype diffraction element of the spectral colorimetric apparatusaccording to Embodiment 1 of the present invention.

FIG. 7 is a principal cross sectional view illustrating a spectralcolorimetric apparatus according to Embodiment 2 of the presentinvention.

FIG. 8 is a principal cross sectional diagram illustrating a color imageforming apparatus according to an embodiment of the present invention.

FIG. 9 is a principal cross sectional view illustrating a conventionalspectral colorimetric apparatus.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention aredescribed in detail with reference to the attached drawings.

Embodiment 1

A spectral colorimetric apparatus according to Embodiment 1 of thepresent invention is described.

FIG. 1 is a perspective view illustrating the spectral colorimetricapparatus using a diffraction element, according to Embodiment 1 of thepresent invention. FIG. 2 is an internal perspective view of thespectral colorimetric apparatus.

In FIG. 2, reflection light (light beam to be tested) diffused by andreflected on a test subject 4 on a detection surface, which isilluminated by an illumination unit including a light source 1 and anillumination optical element 2, is guided to a stop 6 through a guidingoptical element 5. The light having passed through the stop 6 isspectrally condensed by a concave surface reflection type diffractionelement 7 and imaged for each wavelength on a light receiving surface ofa light receiving element array 8 to form a stop image.

The light receiving element array 8 may convert a light quantity foreach wavelength into an electrical signal, to thereby obtain spectralintensity information. The spectral intensity information obtained inthe spectral colorimetric apparatus is transmitted to an analyzingapparatus (not shown). The analyzing apparatus may serve as acolorimetric apparatus for obtaining absolute chromaticity of the testsubject from an internal table based on the input spectral intensityinformation.

In this embodiment, a Rowland type spectral device structure, which is asimple structure and effective for a reduction in size, is employed. Inthe Rowland type spectral device, as illustrated in FIG. 6A, the stop 6and the concave surface reflection type diffraction element 7 areprovided on a Rowland circle 70. Diffraction light for each wavelengthis imaged at a predetermined position on the Rowland circle 70. When thelight receiving element array 8 is provided at the position, a spectralintensity distribution may be simultaneously detected.

As illustrated in FIGS. 1 and 2, for example, desired color patches(patches for chromaticity adjustment) 41, 42, 43, and 44 are printed onthe test subject 4 to be measured. The illumination unit forilluminating the color patches includes the light source 1 and theillumination optical element (optical element having total reflectionsurface and refractive surface) 2. The color patches 41, 42, 43, and 44are sequentially moved in a direction indicated by an arrow Sillustrated in FIG. 1 to be illuminated.

(Light Source)

In this embodiment, a white light emitting diode (white LED), forexample, an ultraviolet LED excitation type white LED, is used as anillumination light source (light source 1).

(Illumination Optical System)

An illumination optical system (illumination optical element) isdescribed with reference to FIGS. 3A and 3B.

An LED generally has a predetermined orientation characteristic based onan element structure thereof. Therefore, an illumination optical element2 is employed to improve illumination efficiency, to thereby uniformlyilluminate the detection surface and reduce a variation in lightquantity due to the rising of the test subject.

The light source 1 is an LED which is generally called a top-view type,which emits light in a direction perpendicular to an electrical mountingboard (not shown) onto which the light source 1 is attached. Theillumination optical element 2 is also attached onto the electricalmounting board to illuminate the detection surface 4. The detectionlight beam which is reflected and diffused on the detection surface 4 isguided to a spectral optical system through a guiding optical system.

The LED serving as the light source 1 has an orientation anglecharacteristic in which a light quantity in a direction close to a planenormal to a light emitting surface is maximum and the light quantitygradually reduces as a tilt from the plane normal increases. Therefore,as illustrated in FIG. 3A, Light Lp1 having a maximum intensity isentered into the illumination optical element located immediately abovethe light source 1 from the vicinity of the center of the light emittingsurface of the light source 1. When an incident surface 2 a of theillumination optical element 2 is tilted in a detection surfacedirection, a light beam entered into the incident surface 2 a isrefracted to become light Lp2. The light having the maximum intensity isbent in the detection surface direction to become light Lp3, is exitedfrom an exit surface 2 b, and reaches the detection surface 4. Anotherlight beam Lp4 exited at a tilt from the plane normal to the lightemitting surface is entered into the incident surface 2 a, and then istotally reflected on a reflective surface 2 c to become light Lp5, isexited from the exit surface 2 b, and reaches the test subject 4.Another light beam Lp6 exited at a tilt from the plane normal in adirection opposite to the travelling direction of the light beam Lp4 isentered into the incident surface 2 a, and then is exited from an exitsurface 2 d to become light Lp7 to reach the detection surface 4.

When the light beam having the maximum intensity is bent at the incidentsurface 2 a of the illumination optical element 2 as described above,the light beam having the maximum intensity may be guided to thedetection surface 4 without tilting the attaching board. Further, thelight beams exited at the angles relative to the light beam having themaximum intensity are guided to the detection surface 4 by the totalreflective surface action and the refractive surface action, to reduce achange in illumination due to a variation in position in a case wherethe test subject 4 flutters on the detection surface 4.

Table 1 illustrates a numerical embodiment with respect to theillumination optical system.

In a coordinate system illustrated in FIGS. 3A and 3B, rotating anglesabout respective X-, Y-, and Z-axes are assumed to be (α, β, γ).

An origin of the coordinate system is set to the center of the detectionsurface.

TABLE 1 Rotating Coordinates angles (°) Detection surface (center) 4 (0,0, 0) (0, 0, 0) Light emitting point center (0, −8, −11) (0, 0, 0)Illumination optical (0, −8, −9.5) (−45, 0, 0) element surface 2aIllumination optical (0, −3.5, −5.5) (−45, 0, 0) element surface 2bIllumination optical (0, −8, −6.5) (60, 0, 0) element surface 2cIllumination optical (0, −2.5, −8) (90, 0, 0) element surface 2dIllumination optical (1.46, −8, −11) (0, −80, 0) element surface 2eIllumination optical (−1.46, −8, −11) (0, 80, 0) element surface 2f

Each of the surfaces of the illumination optical element is flat. Theillumination optical element 2 is a plastic optical element made ofacrylic and manufactured by injection molding.

FIG. 4 is a graph illustrating the light quantities at respectivepositions on the test subject with respect to the center position of therectangular read region 40 on the detection surface 4 in a direction Sof the movement of the test subject 4, with a parameter of deviation ina height direction of the test subject 4 from the detection surface 4 (adirection away from the cover glass 3). The light quantity in a casewhere the test subject is located on the detection surface 4 is set as 1for normalization. As is apparent from the graph, even when the testsubject is fluctuated relative to the position of the detection surface4 by ±0.5 mm, excellent correction is achieved because a variation inlight quantity is smaller than 3%.

In this embodiment, the illumination optical system described above isemployed as a best system. However, the present invention is not limitedto the system. Even when any illumination optical system is employed, aneffect obtained by the present invention is effective.

As illustrated in FIG. 5, the guiding optical element 5 is a light guidematerial for guiding reflection light from the test subject 4 to thestop 6. A read region on the detection surface 4 is defined by the stop6 having a rectangular aperture and is long in a direction perpendicularto the moving direction (arrow S) of the test subject on the detectionsurface 4 and is narrow in the moving direction thereof. The guidingoptical element 5 is a condensing element which exhibits an anamorphicpower having a condensing function in a direction (width direction ofstop 6) parallel to the spectral direction, to thereby form asubstantially linear image by the stop 6.

The detection surface 4 and the stop 6 have substantially a conjugaterelationship in a cross section (width direction of the stop 6) parallelto a spectral direction. Therefore, even when the height position of thetest subject is varied relative to the detection surface 4, a change inlight quantity is small, and hence stable measurement may be achieved.The guiding optical element 5 has a structure to ensure a numericalaperture (NA) equal to or larger than an effective width of a concavesurface reflection type diffraction element 7 of a spectral unit in thestop width direction and to condense light at the front and back of thestop 6, to thereby ensure the amount of movement of the stop 6 duringfocus adjustment.

The light beam to be tested is bent by the guiding optical element in adirection perpendicular to the detection surface 4 to make the detectionsurface and a spectral plane of the spectral unit parallel to eachother, to thereby reduce the thickness of the spectral colorimetricapparatus.

Within the spectral plane, a principal ray of the light beam enters thespectral optical system obliquely with respect to a straight linejoining the center of the light receiving element array 8 with thesurface vertex of the spectral element to make a light receiving surfaceof the light receiving element array 8 and the spectral plane orthogonalto each other, to thereby reduce the thickness of the spectralcolorimetric apparatus.

Note that the spectral plane is defined as a plane including a principalray of an incident light beam on a spectral element (which is a concavesurface reflection type diffraction element in this embodiment, and maybe a transmission type diffraction element or a spectral prism) andrespective principal rays of light beams obtained by diffracting(diffusing) the principal ray of the incident light beam by the spectralelement.

In this embodiment, a composite element having a function of bending theoptical path by total reflection and a function of condensing by arefractive surface is assumed as the guiding optical element. However,the functions may be separated from each other to provide a guidingoptical element including an optical path folding mirror and acondensing lens.

Table 2 illustrates a specific numerical embodiment with respect to theguiding optical element.

In a coordinate system illustrated in FIG. 5, rotating angles aboutrespective axes are expressed by (α, β, γ).

The center of the detection surface is set as an origin of thecoordinate system.

TABLE 2 Curvature radius (mm) Rotating X-Y cross X-Z cross Coordinatesangles (°) section section Detection surface (0, 0, 0) (0, 0, 0) ∞(Flat) (center) 4 Guiding optical (0, 0, −6.0) (0, 0, 0)   3.85(Spherical) element surface 5a Guiding optical (0, 0, −9.0) (0, 45, 0)10.3 ∞ element surface 5b Guiding optical (2.1, 0, −9.0) (0, 90, 0) ∞(Flat) element surface 5c Stop 6 (10, 0, −9.0) (0, 90, 0) ∞ (Flat)

(Spectral Optical System)

As illustrated in FIG. 6A, light passing through the stop 6 isspectrally condensed by the concave surface reflection type diffractionelement 7 and imaged for each wavelength on the light receiving elementarray 8 to form a stop image. FIG. 6A illustrates first-orderdiffraction light L1B, L1G, and L1R and zero-order diffraction light L0.

A Si photo diode array is generally used as the light receiving elementarray 8. Multiple light receiving pixels are laterally arranged on thelight receiving surface of the light receiving element array 8. Each ofthe light receiving pixels may output, as a detection signal, anintensity of the stop image obtained by condensing for each wavelength.The detection signal is detected as a signal corresponding to eachwavelength by a signal processing circuit (not shown), and hencecorresponding chromaticity may be calculated. In view of the structureof the Si photo diode array, the spectral sensitivity is higher close toa near-infrared region and the spectral sensitivity reduces as thewavelength shortens. The signal processing circuit has a configurationcapable of generating a processing signal in consideration of thespectral sensitivity of the light receiving element array 8.

As illustrated in FIG. 6B, the concave surface reflection typediffraction element 7 includes blaze diffraction gratings provided on ananamorphic surface having different curvatures in the Y-direction andthe Z-direction.

In the Rowland type spectral device structure, the concave surfacereflection type diffraction element 7 generally includes a sphericalsurface which serves as a base surface. Therefore, there has been aproblem that, imaging states in the spectral direction and a directionorthogonal to the spectral direction are significantly different fromeach other, and hence large astigmatism occurs to degrade imagingperformance, to thereby deteriorate the resolution of the spectraldevice. This problem occurs in principle and thus may not be completelyeliminated in view of the feature. However, when the curvatures of theconcave surface reflection type diffraction element 7 in the spectraldirection and the direction orthogonal to the spectral direction are setdifferent from each other (that is, the surface of the concave surfacereflection type diffraction element 7 is set an anamorphic surface), animage plane tilt in the direction orthogonal to the spectral directionmay be changed between a short wavelength and a long wavelength.Therefore, necessary and sufficient imaging performance may be obtained.Thus, the base surface of the concave surface reflection typediffraction element is configured as an anamorphic surface.

The concave surface reflection type diffraction element 7 ismanufactured as follows. A plastic optical element is formed byinjection molding and a reflective film made of Al and a high-reflectionfilm made of SiO₂ are formed on the plastic optical element by vapordeposition. The concave surface reflection type diffraction element 7may be manufactured by direct ion beam processing or conventionallithography processing on an optical substrate such as a quartzsubstrate.

The concave surface reflection type diffraction element 7 is describedin detail.

FIG. 6B is a cross sectional diagram illustrating the concave surfacereflection type diffraction element 7 in the spectral direction. A largenumber of fine blaze diffraction gratings 72 are provided on anarc-shaped base surface 71. (Note that the blaze diffraction gratings 72are exaggerated for the purpose of explanation).

Tables 3 and 4 illustrate specifications of the spectral device and theshape of the concave surface reflection type diffraction element 7 inthis embodiment.

Further, Table 5 illustrates the arrangement of the spectral unit. In acoordinate system which corresponds to the coordinate system illustratedin FIG. 6A, rotating angles about respective axes are expressed by (α,β, γ).

An origin of the coordinate system is the center of the stop.

TABLE 3 (Spectral Device Specifications) Spectral range 380 nm to 780 nmLight source Ultraviolet LD excitation white LED Stop width 60 μmDetection element Si photo diode array Diffraction order m 1 Pixel pitchof element 25 μm Wavelength resolution 3.3 nm 

TABLE 4 (Diffraction Element Specifications) Base meridional linecurvature radius [mm] 17.5 Base sagittal line curvature radius [mm]15.45 Grating pitch P [μm] 2.52 Grating height h [μm] 0.25 Incidentangle α [°] 12 Reflective film Al-based multilayer film Effectivediameter [mm] 7

TABLE 5 (Arrangement of Spectral Unit) Rotating Coordinates angles (°)Stop 6 (4.331, 9.881, 0) (0, 0, 0) Concave Surface Reflection Type(17.118, 0, 0) (−12, 0, 0) Diffraction Element 7 Light Receiving ElementArray (0, 0, 0) (−62.6, 0, 0) (center) 8(Cover glass (0.3 mm) and a resin layer (0.45 mm), which are not shown,are provided on a surface of the light receiving element array 8.)

The Rowland type spectral device is used, and hence a Rowland circleradius is 8.75 mm and a curvature radius of a meridional line of theanamorphic base surface of the concave surface reflection typediffraction element 7 within a plane including the Rowland circle isdouble the Rowland circle diameter, that is, 17.5 mm. A curvature radiusof a sagittal line orthogonal to the meridional line 15.45 mm. Aneffective diameter is 7 mm and NA is 0.4.

The concave surface reflection type diffraction element 7 ismanufactured by plastic injection molding using a mold formed bycutting. Alternatively, the concave surface reflection type diffractionelement 7 may be manufactured by directly cutting a base material.Alternatively, the concave surface reflection type diffraction element 7may be manufactured by a replica method of pressing molten glass to amold formed by cutting. In any of the cases, a multilayer film includinga metal film made of Al as a base film is finally formed on the gratingsurface by vapor deposition.

In FIG. 6A, there may be a case where an image of the stop 6 blurs onthe light receiving element array 8 because of disposition precisionamong the stop 6, the concave surface reflection type diffractionelement 7, and the light receiving element array 8 and surface precisionof the concave surface reflection type diffraction element 7. In view ofthis, the stop 6 may be held by a holder so as to be shiftable in alight beam traveling direction. From the same reason, there may be acase where a reaching position of the image of the stop on the lightreceiving element array 8 is deviated. Therefore, the light receivingelement array 8 may be two-dimensionally adjustable (in Y- andZ-directions) on a reference attachment surface perpendicular to thespectral plane, to be adjusted in position.

According to the present invention, the spectral plane of the spectraloptical system is defined by a plane including a principal ray of anincident light beam on an arbitrary spectral element (which is a concavesurface reflection type diffraction element, and may be a transmissiontype diffraction element or a spectral prism in this embodiment) andrespective principal rays of light beams which are obtained bydiffracting (diffusing) the principal ray of the incident light beam bythe spectral element.

When the spectral plane and the detection surface are provided so as tobe parallel to each other, the thickness of the spectral colorimetricapparatus in the direction perpendicular to the detection surface may beeffectively reduced.

When the principal ray of the light beam enters the spectral opticalsystem within the spectral plane obliquely with respect to the straightline joining the center of the light receiving element array 8 with thesurface vertex of the spectral element to make the light receivingsurface of the light receiving element array 8 and the spectral planeorthogonal to each other, the thickness of the spectral colorimetricapparatus may be effectively reduced.

Specifically, the guiding optical system 5 is provided with a functionfor folding an optical path of the reflection light beam from thedetection surface 4 in the direction perpendicular to the normal to thedetection surface 4, to thereby realize the structure in which thedetection surface 4 and the spectral plane are parallel to each other.

The spectral plane may have an angle in a non-dispersion directiondepending on an incident angle on the spectral element. In thisembodiment, the spectral plane of the spectral unit and the detectionsurface are parallel to each other. However, a margin of error ofapproximately ±10° may be allowed without departing from requirements inthis embodiment.

When the thickness of the spectral colorimetric apparatus in thedirection of the plane normal to the detection surface 4 is denoted by dand a maximum width of the spectral colorimetric apparatus in thedirection parallel to the detection surface 4 is denoted by W, it isdesirable to configure the spectral colorimetric apparatus so as tosatisfy the following expression.0.1<d/W<1  (1)

When d/W exceeds an upper limit value of the conditional expression (1),a reduction in thickness may not be practically achieved. When d/Wexceeds a lower limit value of the conditional expression (1), an areanecessary for mounting the spectral colorimetric apparatus becomes toolarge when incorporated in another device, making the spectralcolorimetric apparatus difficult to use. When d/W is within a rangesatisfying the conditional expression (1), the spectral colorimetricapparatus may be effectively reduced in thickness and size.

In this embodiment, the thickness d of the spectral colorimetricapparatus in the direction of the plane normal to the detection surface4 is 15 (mm) and the maximum width W of the spectral colorimetricapparatus within the plane parallel to the detection surface 4 is 40(mm), and hence d/W=0.375. Therefore, d/W satisfies the conditionalexpression (1), and hence a sufficient reduction in thickness may beachieved.

In this embodiment, the illumination optical element which employs thewhite LED as the light source 1 is used to reduce the size of theillumination optical system. The single optical element including thetotal reflection surface and the refractive surface is used as theguiding optical system, to thereby achieve the reduction in size.

In this embodiment, as illustrated in FIG. 2, the longitudinal directionof a rectangular read region 40 on the detection surface 4 which isdefined by the stop 6 is perpendicular to a conveying direction of thetest subject defined within the detection surface 4. Therefore, when acolor patch 41, 42, 43, 44 is assumed as the test subject, a width ofthe color patch in the conveying direction may be reduced. Thus, alarger number of color patches may be formed on a single paper sheet toachieve higher-precision color calibration.

In this embodiment, as illustrated in FIG. 2, the light source 1 and thelight receiving element array 8 are provided in opposite and distantpositions across a reference line 5-5 joining the center of the stop 6with the surface vertex of the concave surface reflection typediffraction element 7, and hence direct flare light from the lightsource may be reduced while the apparatus is reduced in size. The flarelight becomes noise to cause a chromaticity difference duringchromaticity measurement, and hence it is desirable to minimize theflare light.

In this embodiment, the Rowland type spectral device is used for thespectral unit. Even when a transmission type diffraction elementstructure or a general multi-wavelength spectral device structure isemployed for the spectral unit, the spectral unit may be embodiedwithout impairing availability.

The reason why the Rowland type multi-wavelength spectral device is usedin this embodiment is that, the Rowland type multi-wavelength spectraldevice is easily reduced in size by reducing the Rowland circle and maybe made significantly smaller in size than other multi-wavelengthspectral devices.

(Method of Adjusting Spectral Colorimetric Apparatus)

A method of adjusting the spectral colorimetric apparatus is describedwith reference to FIG. 6A.

When the spectral colorimetric apparatus is assembled, it is easy toassume that mere mechanical alignment does not attain excellentperformance, due to a positioning error of the stop 6 and an attachmenterror of the concave surface reflection type diffraction element 7 orthe light receiving element array 8. Therefore, in this embodiment,optical adjustment is performed on two elements, that is, four axes intotal in the diffraction unit.

Hereinafter, a specific element adjusting method is described step bystep.

(First Process)

The stop 6 and the light receiving element array 8 are temporarilyincorporated in a casing and the concave surface reflection typediffraction element 7 is fixed based on a mechanical reference. Theposition of the light receiving element array 8 in the direction of theplane normal to the light receiving surface is regulated and may bemovable only within the plane perpendicular to the plane normal. Lightbeams having specific wavelengths from an external light source 1 aremade incident through the guiding optical element 5. In this embodiment,a light beam having a wavelength of 450 nm and a light beam having awavelength of 650 nm from a spectral light source (having wavelengthresolution of 5 mm) are guided. For each of the wavelengths, the outputof the light receiving element array 8 is observed while the stop 6 ismoved in forward and backward directions along the reference line 5-5joining the center of the stop 6 with the surface vertex of the concavesurface reflection type diffraction element 7. The position of the stop6 that maximizes the output in each of the wavelengths is determined andheld. Therefore, the imaging state on the light receiving element array8 may be adjusted. When the wavelengths are set close to a minimumwavelength and a maximum wavelength in a spectral range of the spectralcolorimetric apparatus, variations in the imaging state is compensatedover the entire region.

In order to perform this process, as described above with reference tothe embodiment, it is important to make a design to reduce variations inlight quantity which is caused by vignetting by the stop 6 in the casewhere the stop 6 is moved in the forward and backward directions by thefunction of the guiding optical system 5. The reason is that, when sucha requirement is not satisfied, a variation in light quantity due to adeviation of focusing and a variation in light quantity due to themovement of the stop 6 may not be distinguished from each other.

(Second Process)

After the completion of the first process, the stop 6 is rotated aboutthe reference line 5-5 to detect the position of the stop 6 at which apeak of the output waveform of the light receiving element array 8 issharper. Therefore, an incidence of a light beam having anotherwavelength to a pixel due to the rotation of the stop 6 or the rotationof the light receiving element array 8 may be reduced. This process maybe executed with only any one of the wavelengths used in the firstprocess.

(Third Process)

After the completion of the second process, the pixel positionadjustment on the light receiving element array 8 is performed with thelight beams of the two wavelengths used in the first process. A lightbeam having the first wavelength is guided and the light receivingelement array 8 is shifted in the spectral direction such that an outputof a specified pixel “A” on the light receiving element array ismaximum. With such a state, a light beam having the second wavelength isguided and a pixel “B” having a maximum output is determined. A simpleRowland type polychromator generally has distortion in view of design,and a pitch of the light beams of the respective wavelengths does notbecome regular on the light receiving element array 8. Therefore, adeviation of a distance between the pixel “A” and the pixel “B” from adesign value is electrically corrected.

(Fourth Process)

After the completion of the third process, a light beam having thecenter wavelength (550 nm in this embodiment) of the spectrum of thespectral colorimetric apparatus is made incident from the external lightsource. The light receiving element array 8 is shifted in the directionorthogonal to the spectral direction to detect, as a best position, aposition in which the light quantity of a pixel corresponding to 550 nmis maximum. Then, the light receiving element array 8 is finally fixed.

A best spectral performance may be obtained through the four adjustmentprocesses described above. The adjusting method is described step bystep. However, the four processes are not necessarily performed in thestated order. The processes may be exchanged or a part of the processesmay be omitted.

Embodiment 2 Example of Spectral Colorimetric Apparatus Using PlanerTransmission Type Diffraction Element

This embodiment illustrates an example of a spectral colorimetricapparatus using a planer transmission type diffraction element for aspectral optical system.

FIG. 7 is a perspective view illustrating a spectral optical systemusing a planer transmission type diffraction element in a spectralcolorimetric apparatus according to Embodiment 2 of the presentinvention.

Embodiment 2 is different from Embodiment 1 in that the spectral elementincluded in the spectral optical system is a planer transmission typediffraction element 107. The illumination optical system and the guidingoptical system are the same as in Embodiment 1.

In this embodiment, a spectral device structure which is a simplestructure and effective for a reduction in size is employed. A detectionoptical system including a stop 106, a condenser lens element 110 fordetection, a planer transmission type diffraction element 107, and animaging lens 111 is configured such that a light beam having apredetermined wavelength is imaged at a predetermined position. When alight receiving element array 108 is provided in the predeterminedposition, a spectral intensity distribution may be obtained at the sametime.

The test subject 4 is illuminated by the same illumination opticalsystem as in Embodiment 1. Reflection light from the illuminated testsubject 4 is guided to the stop 106 through the guiding optical system.Light having passed through the stop 106 is converted into parallellight beams by the condenser lens element 110 for detection. Then, theguided light beam is spectrally separated by the planer transmissiontype diffraction element 107. The guided light beam is condensed by theimaging lens 111 to form a stop image on the light receiving elementarray 108 for each wavelength.

The light receiving element array 108 converts the light quantity ateach wavelength into an electrical signal and transmits the electricalsignal to an analyzing apparatus (not shown). The analyzing apparatusobtains chromaticity of the test subject 4 based on an internal tableand the input spectral intensity information.

The planer transmission type diffraction element 107 includes a largenumber of fine blaze gratings provided on a flat base surface.

Tables 6 and 7 below show specifications of the spectral colorimetricapparatus and the shape of the planer transmission type diffractionelement 107 according to this embodiment.

TABLE 6 (Spectral Colorimetric Apparatus Specifications) Spectral range350 nm to 750 nm Light source Ultraviolet LD excitation white LED Stopwidth 60 μm Detection element Si photo diode array Diffraction order m 1Pixel pitch of element 25 μm Wavelength resolution 3.3 nm Focal lengthof condenser lens 37.1 mm element for detection Focal length of imaginglens 37.1 mm

TABLE 7 (Diffraction Element Specifications) Base meridional linecurvature radius (mm) ∞ Base sagittal line curvature radius (mm) ∞Refractive index of base material 1.539 Grating pitch P (μm) 5.04Grating height h (μm) 0.650 Blaze angle θb (°) 7.3 Incident angle α (°)0

Even in this embodiment, when the same guiding optical system as inEmbodiment 1 is used, the detection surface and the spectral plane maybe made parallel to each other, and hence the thickness in the directionperpendicular to the detection surface may be effectively reduced.

In the system according to this embodiment, the thickness d in thedirection perpendicular to the detection surface is 15 (mm) and themaximum width W within the plane parallel to the detection surface is 80(mm), and hence d/W=0.188. Therefore, d/W satisfies the conditionalexpression (1).

As described above, even in the case of the spectral colorimetricapparatus using the transmission type diffraction element, the thicknessof the spectral colorimetric apparatus in the direction perpendicular tothe detection surface may be effectively reduced.

As described above, even when the general multi-wavelength spectraldevice structure is employed for the spectral colorimetric apparatus,the spectral colorimetric apparatus may be embodied without impairingavailability.

Embodiment 3

FIG. 8 is a principal schematic diagram illustrating a color imageforming apparatus according to an embodiment of the present invention.This embodiment illustrates a tandem type color image forming apparatus,in which four optical scanning apparatuses are arranged to record imageinformation in parallel on surfaces of photosensitive drums each servingas an image bearing member. The color image forming apparatus 160includes optical scanning apparatuses 11, 12, 13, and 14, photosensitivedrums 21, 22, 23, and 24 each serving as the image bearing member,developing devices 31, 32, 33, and 34, an intermediate transferring belt51, and a fixing device 52.

In FIG. 8, respective color signals of R (red), G (green), and B (blue)are input from an external device 53, such as a personal computer, tothe color image forming apparatus 160. The color signals are convertedinto respective image data (dot data) of C (cyan), M (magenta), Y(yellow), and B (black) by a printer controller 54 in the color imageforming apparatus 160. The image data are separately input to theoptical scanning apparatuses 11, 12, 13, and 14. Light beams 41, 42, 43,and 44 which are modulated according to the respective image data areemitted from the optical scanning apparatuses. The photosensitivesurfaces of the photosensitive drums 21, 22, 23, and 24 are scanned withthe light beams in the main scanning direction.

In the color image forming apparatus 160 according to this embodiment,the four optical scanning apparatuses 11, 12, 13, and 14 are arrangedcorresponding to the respective colors of C (cyan), M (magenta), Y(yellow), and B (black). The image signals (image information) arerecorded in parallel on the surfaces of the photosensitive drums 21, 22,23, and 24, to thereby print a color image at high speed.

According to the color image forming apparatus 160 according to thisembodiment, as described above, latent images of the respective colorsare formed on the corresponding surfaces of the photosensitive drums 21,22, 23, and 24 using the light beams based on the respective image datafrom the four scanning optical apparatuses 11, 12, 13, and 14. Afterthat, the latent images are developed by the respective developingdevices and the respective colors are multiply transferred onto theintermediate transport belt 51. Then, the image is transferred to arecording material and formed on the recording material by the fixingdevice 52.

In the color image forming apparatus 160, a colorimetric apparatus 100for chromaticity detection is provided immediately after the fixingdevice 52 on a conveying path of the recording material and facing tothe image forming surface of the recording material. The colorimetricapparatus has the structure described in Embodiment 1. The colorimetricapparatus 100 detects chromaticity of a color patch which is formed onand fixed onto the recording material by the image forming apparatus160. The chromaticity of the color patch fixed onto the recordingmaterial is measured because color matching is then performed in view ofa change in chromaticity due to fixing. The result obtained by thedetection is transferred to the printer controller 54. The printercontroller 54 determines whether or not the output color reproduction ofthe single-color patch is appropriate. When a chromaticity differencebetween the output color reproduction of the single-color patch andchromaticity instructed by the printer controller is within apredetermined range, the color calibration is completed. When thechromaticity difference is outside the predetermined range, the printercontroller performs the color calibration based on the chromaticityinformation until the chromaticity difference falls within thepredetermined range.

As describe above, when the color sensor according to the presentinvention is introduced to an image forming apparatus, a higher degreeof calibration may be performed.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application Nos.2009-110884, filed on Apr. 30, 2009, and 2010-077821, filed on Mar. 30,2010, which are hereby incorporated by reference herein in theirentirety.

What is claimed is:
 1. A spectral colorimetric apparatus for detecting acolor of an image of a test subject, the spectral colorimetric apparatuscomprising: an illumination optical system that illuminates the testsubject located on a detection surface; a spectral optical systemincluding a spectral element that spectrally separates a light beamdiffused by the test subject and a light receiving element array thatdetects a spectral intensity distribution of the light beam spectrallyseparated by the spectral element; and a guiding optical system thatguides a light beam diffused by the test subject to the spectral opticalsystem, wherein a plane, including a principal ray of a light beamincident on the spectral optical system and a principal ray of a lightbeam having been spectrally separated by the spectral element, isdefined as a spectral plane, wherein the detection surface and thespectral plane are parallel to each other, and wherein a light receivingsurface of the light receiving element array and the spectral plane areorthogonal to each other.
 2. A spectral colorimetric apparatus accordingto claim 1, wherein the spectral optical system further comprises a stopfor regulating the light beam diffused by the test subject.
 3. Aspectral colorimetric apparatus according to claim 2, wherein: theguiding optical system bends the light beam diffused by the test subjectin a direction perpendicular to an incident direction to the guidingoptical system, and the detection surface and the stop are conjugatewith each other in at least one cross section.
 4. A spectralcolorimetric apparatus according to claim 2, wherein: a conveyingdirection of the test subject is within a plane including the detectionsurface, and a longitudinal direction of a rectangular read region onthe detection surface, which is defined by the stop, is perpendicular tothe conveying direction of the test subject.
 5. A spectral colorimetricapparatus according to claim 2, wherein: a line joining the stop and asurface vertex of the spectral element is set as a reference line, andthe illumination optical system and the light receiving element arrayare disposed with the reference line sandwiched therebetween.
 6. Aspectral colorimetric apparatus according to claim 2, wherein: thespectral element comprises a concave surface reflection type diffractionelement, and the stop, the concave surface reflection type diffractionelement, and the light receiving element array provided on a Rowlandcircle.
 7. A spectral colorimetric apparatus according to claim 1,wherein the following condition is satisfied:0.1<d/W<1, where d (mm) represents a thickness of the spectralcolorimetric apparatus in a direction of a plane normal to the detectionsurface and W (mm) indicates a maximum width of the spectralcolorimetric apparatus within a plane parallel to the detection surface.8. A color image forming apparatus comprising: a spectral colorimetricapparatus for detecting a color of an image of a test subject, whereinthe spectral colorimetric apparatus comprises: an illumination opticalsystem that illuminates the test subject located on a detection surface;a spectral optical system including a spectral element that spectrallyseparates a light beam diffused by the test subject and a lightreceiving element array that detects a spectral intensity distributionof the light beam spectrally separated by the spectral element; and aguiding optical system that guides a light beam diffused by the testsubject to the spectral optical system, wherein a plane, including aprincipal ray of a light beam incident on the spectral optical systemand a principal ray of a light beam having been spectrally separated bythe spectral element, is defined as a spectral plane, wherein thedetection surface and the spectral plane are parallel to each other, andwherein a light receiving surface of the light receiving element arrayand the spectral plane are orthogonal to each other.